A conventional power supply apparatus receives a DC (direct current) input voltage, and outputs a DC output voltage associated with the input voltage. When the input voltage is greater than a predetermined reference voltage value, the conventional power supply apparatus operates in a first mode to output the input voltage as the output voltage. When the input voltage is less than the reference voltage value, the conventional power supply apparatus operates in a second mode to boost-convert the input voltage into the output voltage and stabilize the output voltage at a predetermined target voltage value. The conventional power supply apparatus directly switches from one of the first and second modes to the other of the first and second modes, and thus endures relatively high current stress. In addition, the conventional power supply apparatus causes each of the input and output voltages to have a relatively large ripple component, and as a result, a system using the conventional power supply apparatus has a relatively short lifetime and relatively poor stability.